L78M thermal characteristics for junction-to-board?

[Dan K.]

Hello,

thermal data of the L78M show junction-to-ambient thermal resistance (θJA) and junction-to-case thermal resistance (θJC) for different packages (e.g., TO-220 and DPAK). Those figures differ, of course. Note that the datasheet provides no junction-to-board thermal resistance (θJB).

I wonder which are the relevant figures for TO-220 and DPAK packages for calculating thermal response:

The TO-220 package is mounted through hole and usually the package's thermal pad has no contact to the board. Thus heat is just just dissipated to surrounding air. Optionally, a heat sink is mounted on the thermal pad. Therefore I assume that the θJA is the relevant figure.

The DPAK package is a surface mounted device. As the thermal pad is soldered to the board, the θJC should be more relevant.

Is that correct? Please advise!

I'd also appreciate if you'd provide an application note that provides a better understanding of thermal resistance data.

Thanks.

[Andrew Neil]

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@Dan K. wrote:

the datasheet provides no junction-to-board thermal resistance (θJB).

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Of course it doesn't - because that is beyond the control or even knowledge of ST.

ST's control & knowledge ends at the case - which is why they give the junction-to-case resistance.

Beyond the case (eg, case-to-board resistance) is entirely down to you & your design.

https://www.google.com/search?q=understanding+thermal+resistance 

[Dan K.]

Thanks Chief,

I know how to use google.